1. Field of the Invention
The present invention concerns a resist curing device in which a resist applied to a silicon wafer is irradiated with light containing ultraviolet rays in the process of semiconductor integrated circuit manufacture.
2. Description of Related Art
In the production of semiconductor integrated circuits, silicon wafers are subjected to patterning, and a resist comprising photosensitive material and base polymer of novolak resin is uniformly applied to the surface of a silicon wafer as pretreatment to form various types of circuits. Photoresists are cured by irradiating silicon wafers with ultraviolet rays while heating the silicon wafer to enhance the heat resistance, chemical resistance and dry-etching resistance of resists.
First, a device such as a silicon wafer is held by vacuum adsorption as a workpiece on the surface of a workpiece stage that is provided with a temperature control means and a workpiece retention means for use in curing of such resists. The workpiece is vacuum adsorbed to uniformly and efficiently transmit heat from the workpiece stage to the workpiece while accurately controlling the temperature. The temperature of the silicon wafer is gradually raised from 100xc2x0 C. to 200xc2x0 C. while irradiating a resist applied to a silicon wafer with light containing ultraviolet rays from an ultraviolet lamp. The temperature is then lowered back to 100xc2x0 C. following ultraviolet irradiation.
Outstanding resist layer characteristics in terms of heat resistance, chemical resistance and dry-etching resistance is obtained by so doing. For example, the heat resistance temperature of a resist whose heat resistance temperature is 140xc2x0 C. can be raised to 250xc2x0 C. by curing with ultraviolet irradiation.
In this manner, a silicon wafer workpiece is vacuum adsorbed on a workpiece stage to accurately control the temperature of the wafer during ultraviolet irradiation. However, the diameter of silicon wafers is large, ranging from 150 mm to 200 mm, and there are cases, depending on the conditions such as the type of resist or the layer thickness, in which such a large silicon wafer must be processed at a rapid heating rate of 1xc2x0 C./second or more. Accordingly, a workpiece stage is formed from metal having good thermal conductivity such as copper to uniformly heat the entire broad surface of a silicon wafer while raising the temperature rapidly. Thus, a great difference in linear expansion coefficients exists between a silicon wafer comprising a single crystal of silicon whose linear expansion coefficient at 20xc2x0 C. to 300xc2x0 C. is 2.5xc3x9710xe2x88x926/K and a workpiece stage of copper whose linear expansion coefficient is 0.334/K.
Accordingly, when a silicon wafer is subjected to rapid rise and drop in temperature while vacuum adsorbed to a workpiece stage, the silicon wafer and workpiece stage rub together due to the great differences in the amount of linear expansion and the amount of heat shrinkage between the two, thereby creating radial rubbing blemishes on the backside of the silicon wafer.
Furthermore, the surface of a workpiece stage may be coated with ceramic to prevent direct contact between a silicon wafer and a metal workpiece stage in consideration of metal contamination of a silicon wafer. But even if the surface of a ceramic coating layer is adequately polished, it would remain rough compared to the surface of metal and many rubbing blemishes would form on the backside of the silicon wafer. The number of such rubbing blemishes could amount to 5000 to 15,000 depending on conditions, and their length could extend to the edges of a silicon wafer, reaching 0.5 mm.
The dislocation of the silicon single crystal could develop in the course of heating in subsequent steps if such rubbing blemishes develop on the backside of silicon wafers, and sleeves could develop in silicon wafers as such dislocations grow, resulting in production rejects. Furthermore, dust of shaved silicon powder that is created when such rubbing blemishes develop sticks to the backside of silicon wafers and is fed to the subsequent steps. For this reason, the wash in the subsequent washing process as well as the etching liquid in the etching process become contaminated with dust which is a cause of production rejects.
Thus, the primary purpose of the present invention is to provide a resist curing device in which rubbing blemishes do not develop on the backside of a silicon wafer even when a silicon wafer coated with a resist is vacuum adsorbed onto a workpiece stage and is subjected to curing processing by ultraviolet irradiation while being heated and cooled.
To attain such purposes, the present invention provides a resist curing device with a workpiece stage comprising a light source unit that carries out ultraviolet irradiation, a workpiece retention means that vacuum adsorbs a silicon wafer laid thereupon as a workpiece, and a workpiece temperature control means that heats and cools the workpiece, wherein the resist applied to the workpiece is subjected to ultraviolet irradiation and cured while the workpiece that is held on the surface of the workpiece stage is heated, the resist curing device further including a plate composed preferably of Si (silicon), SiO2 (quartz), SiC (silicon carbide), or Si3N4 (silicon nitride) with through-holes to permit vacuum adsorption from the workpiece stage to act on the workpiece. The plate is interposed between the workpiece and the upper surface of the workpiece stage, and ultraviolet irradiation is carried out while the plate and workpiece are vacuum adsorbed on the workpiece stage by the workpiece retention means.